1. Field of the Invention
The present invention relates to an image forming system of an electrophotographic printer and a method of forming an image using the same.
2. Description of the Related Art
As shown in FIG. 1, a conventional image forming system of an electrophotographic printer (such as a color laser printer or the like) includes four photosensitive drums Dy, Dm, Dc, and Dk, which correspond to four colors such as yellow, magenta, cyan, and black, a charger 11 to charge the photosensitive drums Dy, Dm, Dc, and Dk, and an exposing unit 12 to radiate light onto each of the charged photosensitive drums Dy, Dm, Dc, and Dk and to form a desired electrostatic latent image thereon. The conventional apparatus further includes a developing unit 13 to develop the electrostatic latent image with a developer for each of the four colors, a transfer belt 14 on which the images having the four colors developed on the photosensitive drums Dy, Dm, Dc, and Dk are sequentially overlapped and a complete color image is formed, and a transfer unit 16 to transfer the image having the four overlapped colors on the transfer belt 14 onto a paper P. Thus, in order to print the desired color image, an image for each of the four colors is developed on the four photosensitive drums Dy, Dm, Dc, and Dk, and each image is sequentially overlapped on the transfer belt 14, thereby forming a final color image, which then is printed onto the paper P by the transfer unit 16.
The transfer unit 16 includes a paper transfer roller 16a and a paper transfer backup roller 16b to closely press the paper P against the transfer belt 14, and a potential applying unit 16c to apply a voltage to the paper transfer backup roller 16b and to apply an electric force so that the image formed on the transfer belt 14 is transferred onto the paper P. In FIG. 1, the developer used to form an image is positively charged. The potential applying unit 16c applies a positive voltage to the paper transfer backup roller 16b so that the image is transferred onto the paper P by an electric repulsive force. That is, when the image developed on the photosensitive drums Dy, Dm, Dc, and Dk are transferred onto the transfer belt 14, a negative voltage is applied to a corresponding backup roller 15 of each of the photosensitive drums Dy, Dm, Dc, and Dk so that a toner image charged positively by an electric repulsive force is transferred onto the transfer belt 14. When the image is transferred onto the paper P, a positive voltage is applied to the paper transfer backup roller 16b so that the toner image is transferred onto the paper P from the transfer belt 14 by a repulsive force.
The entire image formed on the transfer belt 14 is transferred onto the paper P. However, in the above structure, the image formed on the transfer belt 14 is transferred onto the paper P by an electric force caused by a voltage applied to the paper transfer backup roller 16b, and thus the developer that is not transferred onto the transfer belt 14 remains after this transfer operation. Of course, pressure which acts to closely press the paper P to the transfer belt 14 is generated by the paper transfer roller 16a and the paper transfer backup roller 16b. However, this pressure is less than 3 kgf and only acts to closely attach the paper P to the transfer belt 14.
Meanwhile, a method to increase the attaching pressure has been proposed in order to improve the image transfer efficiency. In this case, even if the transfer efficiency is increased slightly, the toner remaining on the transfer belt 14 is too strongly attached to the transfer belt 14 such that the toner cannot be properly cleaned, and thus the lifespan of the transfer belt 14 is reduced. The concentration of the developer may be reduced to reduce the amount of the toner of the image formed on the transfer belt 14. However, in this case, when the image is compressed between the photosensitive drums Dy, Dm, Dc, and Dk and the backup roller 15, and between the paper transfer roller 16a and the paper backup transfer roller 16b, a liquid carrier that is contained in the image is squeezed and flows out, and thus the image spreads excessively.
Also, in the case of a color image, the toner having the four colors such as yellow, magenta, cyan, and black, is sequentially overlapped on the transfer belt 14 by an electric force generated in the developing unit 13 for each color, and a color image is formed. Hence, in this procedure, the electric force which acts to attach the toner to the transfer belt 14, is accumulated gradually and increased, and thus the color image may not be properly transferred onto the paper P.
Thus, an image forming system which solves these problems and more stably performs a transfer operation from the photosensitive drums Dy, Dm, Dc, and Dk onto the paper P through the transfer belt 14 is needed.